As long as the push-button switch S1 is pressed,
the multivibrator remains quiet.
The LED is out.

By contrast, if you unpress S1,
the LED1 starts to blink.

In a real world application, the push-button switch
could be replaced by a trip wire
or a contact at a door.
When the trip wire is broken or the door opened,
it is indicated by the flashing light of the LED.